Fuel cell

ABSTRACT

A fuel cell stack comprising a second metal separator set to have an external dimension larger than a first metal separator, wherein the second metal separator comprises, formed integrally, a first seal member in contact with the peripheral edge of a first electrolyte membrane/electrode structure, a second seal member in contact with the peripheral edge of the first metal separator, and a third seal member in contact with the peripheral edge of an adjoining fourth metal separator. Since the first seal member, the second seal member and the third seal member are integrally formed on one surface of the second separator or one surface of the first separator, a seal-forming step can be carried out at one effort, simply and economically. In addition, use of a triple seal structure containing the first through the third seal members can favorably improve the sealing feature of reaction gas and minimize reaction gas leakage.

RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.12/299,283, filed 31 Oct. 2008, which is a 35 U.S.C. 371 national stagefiling of International Application No. PCT/JP2007/059311, filed 1 May2007, which claims priority to Japanese Patent Application No.2006-127578, filed 1 May 2006, Japanese Patent Application No.2006-274165, filed 5 Oct. 2006, Japanese Patent Application No.2006-333305, filed 11 Dec. 2006, and Japanese Patent Application No.2007-030448, filed 9 Feb. 2007, all filed in Japan. This application isalso related to Japanese Patent Application No. 2007-119308 filed 27Apr. 2007. The contents of the aforementioned applications are herebyincorporated by reference.

TECHNICAL FIELD

The present invention relates to a fuel cell including an electrolyteelectrode assembly and first and second separators sandwiching theelectrolyte electrode assembly. The electrolyte electrode assemblyincludes electrodes and an electrolyte interposed between theelectrodes.

BACKGROUND ART

For example, a solid polymer electrolyte fuel cell employs a solidpolymer electrolyte membrane. The solid polymer electrolyte membrane isa polymer ion exchange membrane. In the fuel cell, the solid polymerelectrolyte membrane is interposed between an anode and a cathode eachcomprising an electrode catalyst layer and porous carbon to form amembrane electrode assembly (electrolyte electrode assembly). Themembrane electrode assembly is sandwiched between separators (bipolarplates) to form a unit cell. In use, normally, a predetermined number ofunit cells are stacked together to form a fuel cell stack.

In general, the fuel cell adopts so called internal manifold structurewhere supply passages and discharge passages extend through theseparators in the stacking direction. The fuel gas, theoxygen-containing gas, and the coolant are supplied from the respectivesupply passages to a fuel gas flow field, an oxygen-containing gas flowfield, and a coolant flow field, and then, discharged to the respectivedischarge passages.

Therefore, in the fuel cell, it is necessary to prevent leakage of thefuel gas, the oxygen-containing gas, and the coolant individually. Inthis regard, for example, a fuel cell disclosed in Japanese Laid-OpenPatent Publication No. 2002-270202 is known. In the fuel cell stack, asshown in FIG. 45, fuel cells 1001 are stacked together, and each of thefuel cells 1001 is formed by sandwiching an electrode assembly 1002between a first separator 1003 and a second separator 1004.

The electrode assembly 1002 includes an anode 1002 b, a cathode 1002 c,and an electrolyte membrane 1002 a interposed between the anode 1002 band the cathode 1002 c. The surface area of the cathode 1002 c issmaller than the surface area of the anode 1002 b. The first separator1003 and the second separator 1004 are hermetically sealed by an outerseal member 1005 a, and a space between the second separator 1004 andthe outer end of the electrode assembly 1002 are hermetically sealed byan inner seal member 1005 b. Further, a seal member 1005 c is providedbetween the fuel cells 1001.

As shown in FIG. 46, in the fuel cell 1001 disclosed in JapaneseLaid-Open Patent Publication No. 2002-270202, a fuel gas inlet 1007 a,an oxygen-containing gas inlet 1008 a, and a coolant inlet 1009 a areformed at one end of first and second separators 1003, 1004 in alongitudinal direction, and a fuel gas outlet 1007 b, anoxygen-containing gas outlet 1008 b, and a coolant outlet 1009 b areformed at the other end of the first and second separators 1003, 1004 inthe longitudinal direction.

In the fuel cell 1001, the dimensions of the first and second separators1003, 1004 are relatively large in comparison with the outer dimensionsof the electrode assembly 1002. In the case of adopting a structurewhere several hundreds of the fuel cells 1001 are stacked together andused as the fuel cell stack in a vehicle application, the overall sizeand weight of the fuel cell stack may become large undesirably.

Further, in the fuel cell 1001, an outer seal member 1005 and an innerseal member 1006 having desired shapes are formed beforehand, and then,the outer seal member 1005 and the inner seal member 1006 are supported,e.g., by the second separator 1004. Thus, the process of producing thefuel cell 1001 and operation of assembling the fuel cell 1001 may becomecomplicated undesirably.

Further, in a fuel cell disclosed in Japanese Laid-Open PatentPublication No. 2002-025587, as shown in FIG. 47, a fuel unit cell 1101is sandwiched between a first separator 1102 and a second separator1103. The fuel unit cell 1101 is formed by sandwiching a solid polymerelectrolyte membrane 1104 between a cathode 1105 and an anode 1106. Thecathode 1105 and the anode 1106 include gas diffusion layers 1105 a,1106 a.

The solid polymer electrolyte membrane 1104 protrudes out from innercircumferences of the cathode 1105 and the anode 1106. The surface areaof the cathode 1105 is smaller than the surface area of the anode 1106.

A first seal 1107 a and a second seal 1107 b are attached between thefirst separator 1102 and the second separator 1103. The first seal 1107a tightly contacts the solid polymer electrolyte membrane 1104, and thefirst seal 1107 a is provided around the cathode 1105. The second seal1107 b is provided around the anode 1106, and around the first seal 1107a. Thus, leakage of the oxygen-containing gas is prevented by the firstseal 1107 a, and leakage of the fuel gas is prevented by the second seal1107 b. In the structure, the first seal 1107 a and the second seal 1107b are provided at positions deviated laterally with respect to thestacking direction of the fuel cell. Therefore, reduction in the overallthickness of the fuel cell in the stacking direction is achieved.

In the case of the fuel unit cell 1101, at the time of stacking aplurality of the fuel unit cells 1101, a coolant flow field is formedbetween each fuel unit cell 1101 along the electrode surface for coolingthe fuel unit cell 1101. Thus, a seal member for preventing leakage ofthe coolant needs to be provided between each fuel unit cell 1101. Inthe presence of the seal member, the fuel unit cells 1101 tend to bespaced from each other, and reduction in the overall size of the fuelcell stack may not be achieved.

In a process control apparatus disclosed in Japanese Laid-Open PatentPublication No. 06-218275, as shown in FIG. 48, stack plates each formedby overlapping two plates in parallel with each other, and units 1202are stacked alternately. The unit 1202 is formed by sandwiching an MEA1202 a between an anode 1202 b and a cathode 1202 c, and sandwichingthese components between a pair of contact plates 1202 d.

A first chamber 1203 a is formed between the plate 1201 a and the unit1202, a second chamber 1203 b is formed between the plate 1201 b and theunit 1202, and a third chamber 1203 c is formed between the plates 1201a, 1201 b. A passage 1205 extends through ends of the plates 1201 a,1201 b through packings 1204.

The passage 1205 is connected to, e.g., the second chamber 1203 bthrough a flow field 1206 between the plates 1201 a, 1201 b. Though notshown, two other passages extend in the stacking direction, and theother two passages are connected to the first chamber 1203 a and thethird chamber 1203 c, respectively through flow fields (not shown)between the plates 1201 a, 1201 b.

However, in the process control apparatus, the flow field 1206 forconnecting the passage 1205 extending in the stacking direction to thesecond chamber 1203 b needs to have the sufficient flow field height inthe stacking direction, and the sufficient seal height by the packing1204, in order for sufficient fluid to flow therethrough. Thus, thespace between the units 1202 becomes considerably large, and reductionin the overall size of the fuel cell cannot be achieved.

In particular, a fuel cell stack mounted in a vehicle is formed bystacking a large number of, e.g., several hundreds of fuel cells.Therefore, space between the fuel cells becomes large, and reduction inthe overall size of the fuel cell cannot be achieved.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a fuel cell which makesit possible to perform, in particular, seal forming operationeconomically and efficiently, while effectively improving theperformance of preventing leakage of reactant gases.

Another object of the present invention is to provide a fuel cell whichmakes it possible to suitably reduce the overall weight and size of thefuel cell, in particular, even if a larger number of components arestacked to form the fuel cell.

Still another object of the present invention is to provide a fuel cellwhich makes it possible to simplify the structure of the fuel cell, and,in particular, economically and efficiently perform seal formingoperation.

Still another object of the present invention is to provide a fuel cellwhich makes it possible to suitably prevent leakage of a fuel gas, anoxygen-containing gas, and a coolant, and reduce the thickness in thestacking direction as much as possible.

Still another object of the present invention is to provide a fuel cellin which reactant gases suitably flow between reactant gas passagesextending in the stacking direction and reactant gas flow fieldsextending along the electrode surfaces, and which makes it possible toreduce the thickness in the stacking direction.

The present invention relates to a fuel cell including an electrolyteelectrode assembly and first and second separators sandwiching theelectrolyte electrode assembly. The electrolyte electrode assemblyincludes a first electrode, a second electrode, and an electrolyteinterposed between the first electrode and the second electrode. Thesecond electrode has a surface area smaller than that of the firstelectrode.

The first separator faces the first electrode, the second separator hasouter dimensions different from those of the first separator, and facesthe second electrode. A first seal member, a second seal member, and athird seal member are formed integrally on one surface of the secondseparator or on one surface of the first separator. The first sealmember contacts the electrolyte at an outer end of the electrolyteelectrode assembly. The second seal member contacts an outer end of thefirst separator or the second separator. A third seal member contacts anouter end of the adjacent second separator or an outer end of theadjacent first separator.

Further, in a fuel cell according to the present invention, outerdimensions of the first separator are smaller than those of the secondseparator, and fluid passages at least including a fuel gas supplypassage, a fuel gas discharge passage, an oxygen-containing gas supplypassage, an oxygen-containing gas discharge passage extend through anouter end of the second separator in the stacking direction, atpositions outside the outer end of the first separator.

Further, in a fuel cell according to the present invention, outerdimensions of the second metal separator are larger than those of thefirst metal separator, metal of the first metal separator is exposedover the entire surface, and a seal member is formed integrally only onthe second metal separator.

The seal member is formed on one surface of the second metal separatorfacing the electrode. The seal member includes an inner seal contactingan outer end of the electrolyte electrode assembly, and an outer sealcontacting an outer end of the adjacent second metal member.

Further, in a fuel cell according to the present invention, at least thefirst separator or the second separator includes a first seal forpreventing leakage of the fuel gas, a second seal for preventing leakageof the oxygen-containing gas, and a third seal for preventing leakage ofthe coolant. The first seal, the second seal, and the third seal areoffset from one another in the stacking direction.

Further, in a fuel cell according to the present invention, theelectrolyte electrode assembly has an uneven portion in the same planeas the electrode surface, and the uneven portion forms a connectionchannel connecting the reactant gas flow field to the reactant gaspassage. Uneven portions of adjacent electrolyte electrode assemblies inthe stacking direction are offset from each other in the stackingdirection.

Further, in a fuel cell according to the present invention, the firstelectrolyte electrode assembly has a first uneven portion in the sameplane as the electrode surface, and the first uneven portion forms afirst connection channel connecting the reactant gas flow field forsupplying the reactant gas along the electrode surface, to the reactantgas passage. The second electrolyte electrode assembly has a seconduneven portion in the same plane as the electrode surface, and thesecond uneven portion forms a second connection channel connecting thereactant gas flow field for supplying the reactant gas along theelectrode surface, to the reactant gas passage. The first uneven portionand the second uneven portion are offset from each other in the stackingdirection.

In the present invention, the first seal member, the second seal member,and the third seal member are formed integrally on one surface of thesecond separator or on one surface of the second separator. Thus, theseal forming process is significantly simplified, and carried outeconomically. Further, by adopting triple seal structure including thefirst to third seal members, improvement in the reactant gas sealingperformance is achieved suitably, and leakage of the reactant gas isprevented as much as possible.

Further, the outer dimensions of one of the first separator and thesecond separator are relatively small in comparison with the outerdimensions of the other of the first separator and the second separator.Thus, the desired rigidity at the time of stacking the fuel cells, andthe rigidity of the electrode surface required for assembling the fuelcells are maintained. Further, the weight reduction of the fuel cell isachieved, and the overall weight of the fuel cell stack is reduced.

Further, in the present invention, the outer dimensions of the firstseparator are smaller than the outer dimensions of the second separator.The fluid passages pass through only the second separator. Thus, thesize of the first separator is reduced as much as possible, and weightreduction is achieved easily. Therefore, in particular, at the time ofstacking a large number of fuel cells to form a fuel cell stack, theoverall size and weight of the fuel cell stack are reduced considerably.

Further, in the present invention, the seal members are formedintegrally with the second metal separator. Thus, sealing structure issimplified significantly. In comparison with the structure in which theseal members having a desired shape are formed beforehand, theperformance of assembling the fuel cell is improved suitably. It isbecause operation of positioning the seal members relative to the secondmetal separator is not required at the time of assembling the fuel cell.

Further, operation of forming the seal members is not required for thefirst metal separator, and operation of forming the seal members is onlyrequired for the second metal separator. Thus, seal forming operation isperformed economically and efficiently. Reduction in the overall cost ofproducing the fuel cell is achieved easily.

Further, in the present invention, the first seal for preventing leakageof the fuel gas, the second seal for preventing leakage of theoxygen-containing gas, and the third seal for preventing leakage of thecoolant are not overlapped with each other in the stacking direction.Thus, the sufficient seal height is achieved in each of the first seal,the second seal, and the third seal, and it is possible to suitablyprevent leakage of the fuel gas, the oxygen-containing gas, and thecoolant, and reduce the thickness in the stacking direction.Accordingly, the overall size of the fuel cell is reduced easily andreliably.

Further, in the present invention, the uneven portions provided in theelectrolyte electrode assemblies adjacent to each other in the stackingdirection are offset from each other in the stacking direction.Therefore, the adjacent channels are not overlapped with each other inthe stacking direction. Thus, it is possible to reduce the thickness inthe stacking direction in each unit cell, while maintaining thesufficient flow field height and seal height in the channel connectingthe reactant gas flow field and the reactant gas passages. Accordingly,the overall size of the fuel cell is reduced easily, and in particular,the size of the fuel cell stack mounted in the vehicle is reducedsuitably.

Further, in the present invention, the first electrolyte electrodeassembly and the second electrolyte electrode assembly are adjacent toeach other in the stacking direction, and the first uneven portion ofthe first electrolyte electrode assembly and the second uneven portionof the second electrolyte electrode assembly are offset from each otherin the stacking direction. Therefore, the adjacent first channel andsecond channel are not overlapped with each other in the stackingdirection. Thus, it is possible to reduce the thickness of the firstunit cell and the second unit cell in the stacking direction whilemaintaining the sufficient flow field height and seal height in thefirst channel and the second channel connecting the reactant gas flowfield to the reactant gas passages. Accordingly, the overall size of thefuel cell is reduced easily, and in particular, the size of the fuelcell stack mounted in the vehicle is reduced suitably.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic perspective view showing a fuel cell according toa first embodiment of the present invention.

FIG. 2 is a cross sectional view showing the fuel cell.

FIG. 3 is an exploded perspective view showing unit cells of the fuelcell.

FIG. 4 is a view showing one surface of a first metal separator.

FIG. 5 is a view showing one surface of a second metal separator.

FIG. 6 is a view showing the other surface of the second metalseparator.

FIG. 7 is a view showing one surface of a fourth metal separator.

FIG. 8 is a view showing the other surface of the fourth metalseparator.

FIG. 9 is a view showing a flow of an oxygen-containing gas in the fuelcell.

FIG. 10 is a view showing another flow of the oxygen-containing gas inthe fuel cell.

FIG. 11 is a view showing a flow of a fuel gas in the fuel cell.

FIG. 12 is a view showing another flow of the fuel gas in the fuel cell.

FIG. 13 is a view showing a flow of a coolant in the fuel cell.

FIG. 14 is a view showing another flow of the coolant in the fuel cell.

FIG. 15 is a partial cross sectional view showing a fuel cell accordingto a second embodiment of the present invention.

FIG. 16 is a partial cross sectional view showing a fuel cell accordingto a third embodiment of the present invention.

FIG. 17 is a cross sectional view showing a fuel cell according to afourth embodiment of the present invention.

FIG. 18 is an exploded perspective view showing the fuel cell.

FIG. 19 is a view showing one surface of a second metal separator of thefuel cell.

FIG. 20 is a cross sectional view showing a fuel cell according to afifth embodiment of the present invention.

FIG. 21 is a cross sectional view showing a fuel cell according to asixth embodiment of the present invention.

FIG. 22 is an exploded perspective view showing the fuel cell.

FIG. 23 is a view showing one surface of a first metal separator of thefuel cell.

FIG. 24 is a view showing the other surface of the first metalseparator.

FIG. 25 is a view showing one surface of a second metal separator of thefuel cell.

FIG. 26 is a cross sectional view showing a fuel cell according to aseventh embodiment of the present invention.

FIG. 27 is a cross sectional view showing a fuel cell according to aneighth embodiment of the present invention.

FIG. 28 is an exploded perspective view showing a fuel cell according toa ninth embodiment of the present invention.

FIG. 29 is an exploded perspective view showing a fuel cell according toa tenth embodiment of the present invention.

FIG. 30 is an exploded perspective view showing a fuel cell according toan eleventh embodiment of the present invention.

FIG. 31 is a cross sectional view showing a fuel cell according to atwelfth embodiment of the present invention.

FIG. 32 is an exploded perspective view showing a unit cell of the fuelcell.

FIG. 33 is a view showing one surface of a second metal separator of theunit cell.

FIG. 34 is a cross sectional view showing a fuel cell according to athirteenth embodiment of the present invention.

FIG. 35 is an exploded perspective view showing a unit cell of the fuelcell.

FIG. 36 is a front view showing a first metal separator of the unitcell.

FIG. 37 is a view showing one surface of a second metal separator of theunit cell.

FIG. 38 is a cross sectional view showing a fuel cell according to afourteenth embodiment of the present invention.

FIG. 39 is an exploded perspective view showing a unit cell of the fuelcell.

FIG. 40 is a cross sectional view showing a fuel cell according to afifteenth embodiment of the present invention.

FIG. 41 is an exploded perspective view showing a unit cell of the fuelcell.

FIG. 42 is a cross sectional view showing a fuel cell according to asixteenth embodiment of the present invention.

FIG. 43 is an exploded perspective view showing a unit cell of the fuelcell.

FIG. 44 is a cross sectional view showing a fuel cell stack according toa seventeenth embodiment of the present invention.

FIG. 45 is a view showing a conventional fuel cell stack.

FIG. 46 is a view showing the fuel cell.

FIG. 47 is a view showing a conventional fuel cell.

FIG. 48 is a view showing a conventional process control apparatus.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a perspective view schematically showing a fuel cell 10according to a first embodiment of the present invention. FIG. 2 is across sectional view showing the fuel cell 10.

The fuel cell 10 includes a stack body 14 formed by stacking unit cells12 a, 12 b alternately in a horizontal direction indicated by an arrowA. End plates 16 a, 16 b are provided at opposite ends of the stack body14 in the stacking direction. Though components between the end plates16 a, 16 b are tightened together by tie-rods (not shown) in theembodiment, for example, the entire stack body 14 may be placed in acasing (not shown).

The unit cell 12 a is formed by sandwiching a first membrane electrodeassembly (electrolyte electrode assembly) 20 a between a first metalseparator 22 and a second metal separator 24. The unit cell 12 b isformed by sandwiching a second membrane electrode assembly 20 b betweena third metal separator 26 and a fourth metal separator 28. The unitcells 12 a, 12 b are oppositely oriented to each other at an angle of180° on a plane of the unit cells 12 a, 12 b. In practice, the secondmembrane electrode assembly 20 b is the same as the first membraneelectrode assembly 20 a, the third metal separator 26 is the same as thefirst metal separator 22, and the fourth metal separator 28 is the sameas the second metal separator 24.

As shown in FIG. 3, the first membrane electrode assembly 20 a comprisesa cathode (first electrode) 32 a, an anode (second electrode) 34 a, anda solid polymer electrolyte membrane (electrolyte) 30 a interposedbetween the cathode 32 a and the anode 34 a. The solid polymerelectrolyte membrane 30 a is formed by impregnating a thin membrane ofperfluorosulfonic acid with water, for example. The surface area of thecathode 32 a is larger than the surface area of the anode 34 a. Thecathode 32 a is provided over the entire surface of the solid polymerelectrolyte membrane 30 a (referred to as a stepped MEA).

Each of the cathode 32 a and the anode 34 a has a gas diffusion layer(not shown) such as a carbon paper, and an electrode catalyst layer (notshown) of platinum alloy supported on surfaces of porous carbonparticles. The carbon particles are deposited uniformly on the surfaceof the gas diffusion layer. Each of the cathode 32 a and the anode 34has an area (catalyst area) 36 a for applying a catalyst.

The first membrane electrode assembly 20 a has a substantiallyquadrangular shape as a whole. At opposite ends of the first membraneelectrode assembly 20 a in the direction indicated by the arrow B, firstuneven portions 38 a are provided in the same plane as the electrodesurface. The first uneven portion 38 a at one end of the first membraneelectrode assembly 20 a in the direction indicated by the arrow Bincludes a first extension 37 a, a first cutout 39 a, a second extension37 b, and a second cutout 39 b from upper to lower positions. Further,the first uneven portion 38 a at the other end of the first membraneelectrode assembly 20 a in the direction indicated by the arrow Bincludes a third extension 37 c, a third cutout 39 c, a fourth extension37 d, and a fourth cutout 39 d from upper to lower positions.

The outer dimensions of the first metal separator 22 are smaller thanthe outer dimensions of the second metal separator 24. As shown in FIGS.3 and 4, the first metal separator 22 has a first oxygen-containing gasflow field (first reactant gas flow field) 40 on a surface 22 a facingthe first membrane electrode assembly 20 a. The first oxygen-containinggas flow field (first reactant gas flow field) 40 is formed in an areacorresponding to the catalyst area 36 a of the first membrane electrodeassembly 20 a. By forming ridges 40 a protruding on the side of thesurface 22 a and grooves 40 b alternately, the first oxygen-containinggas flow field 40 extends straight in the direction indicated by thearrow B. Embossed sections 40 c are formed on both sides of the firstoxygen-containing gas flow field 40. At one end of the firstoxygen-containing gas flow field 40 in the direction indicated by thearrow B, a corrugated inlet section 41 a is formed, and likewise, at theother end of the first oxygen-containing gas flow field 40, a corrugatedoutlet section 41 b is formed. As shown in FIG. 4, the inlet section 41a and the outlet section 41 b protrude from the outer end of the firstmembrane electrode assembly 20 a, respectively.

The first metal separator 22 has an outer shape forming section 42having a desired uneven shape. The outer shape forming section 42 islarger than the outer size of the first membrane electrode assembly 20a. By forming the first oxygen-containing gas flow field 40 on thesurface 22 a of the first metal separator 22, a first coolant flow field44 is formed on the back of the corrugated surface 22 a.

As shown in FIG. 3, the second metal separator 24 has a laterallyelongated rectangular shape. At one end of the second metal separator 24and the fourth metal separator 28 in the direction indicated by thearrow B, an oxygen-containing gas supply passage 46 a for supplying anoxygen-containing gas (one reactant gas), a coolant supply passage 48 afor supplying a coolant, and a fuel gas discharge passage 50 b fordischarging a fuel gas (other reactant gas), such as ahydrogen-containing gas, are arranged vertically in a directionindicated by an arrow C. The oxygen-containing gas supply passage 46 a,the coolant supply passage 48 a, and the fuel gas discharge passage 50 bextend through the second metal separator 24 and the fourth metalseparator 28 in the direction indicated by the arrow A.

At the other end of the second metal separator 24 and the fourth metalseparator 28 in the direction indicated by the arrow B, a fuel gassupply passage 50 a for supplying the fuel gas, a coolant dischargepassage 48 b for discharging the coolant, and an oxygen-containing gasdischarge passage 46 b for discharging the oxygen-containing gas arearranged vertically in the direction indicated by the arrow C. The fuelgas supply passage 50 a, the coolant discharge passage 48 b, and theoxygen-containing gas discharge passage 46 b extend through the secondmetal separator 24 and the fourth metal separator 28 in the directionindicated by the arrow A.

As shown in FIG. 5, the second metal separator 24 has a first fuel gasflow field (second reactant gas flow field) 52 on a surface 24 a facingthe first membrane electrode assembly 20 a. The first fuel gas flowfield 52 is formed in an area corresponding to the catalyst area 36 a ofthe first membrane electrode assembly 20 a. By forming ridges 52 aprotruding on the side of the surface 24 a and grooves 52 b alternately,the first fuel gas flow field 52 extends in the direction indicated bythe arrow B. Embossed sections 52 c are formed on both sides of thefirst fuel gas flow field 52.

As shown in FIG. 6, the second metal separator 24 has a second coolantflow field 54 on a surface 24 b, i.e., on the back of the surface 24 aforming the first fuel gas flow field 52. At one end of the secondcoolant flow field 54 in the direction indicated by the arrow B, acorrugated inlet section 56 a is formed, and likewise, at the other endof the second coolant flow field 54 in the direction indicated by thearrow B, a corrugated outlet section 56 b is formed.

When the second metal separator 24 and the third metal separator 26 arestacked together, the inlet section 56 a and the outlet section 56 b areprovided at cutouts of the third metal separator 26. The coolant supplypassage 48 a is connected to the second coolant flow field 54 throughthe inlet section 56 a and the coolant discharge passage 48 b isconnected to the second coolant flow field 54 through the outlet section56 b.

The second metal separator 24 has two fuel gas inlet holes 58 a adjacentto the fuel gas supply passage 50 a, and two fuel gas outlet holes 58 badjacent to the fuel gas discharge passage 50 b. Further, the secondmetal separator 24 has three oxygen-containing gas inlet holes 60 a nearthe oxygen-containing gas supply passage 46 a, and threeoxygen-containing gas outlet holes 60 b near the oxygen-containing gasdischarge passage 46 b.

As shown in FIGS. 2 and 5, a seal member 62 is formed integrally on asurface 24 a of the second metal separator 24, around the first fuel gasflow field 52. The seal member 62 includes a first seal (first sealmember) 62 a as an inner seal formed integrally with the second metalseparator 24, a second seal (second seal member) 62 b positioned outsidethe first seal 62 a, and a third seal (third seal member) 62 c as anouter seal positioned outside the second seal 62 b. The seal member 62is made of an EPDM (Etylene Propylene rubber), a silicone rubber, anitrile rubber, or an acrylic rubber. For example, the seal member 62 isformed by injection molding using melted resin obtained by heatingsilicone resin to a predetermined temperature (e.g., 160° C. to 170°C.).

The first seal member 62 a contacts the outer end of the first membraneelectrode assembly 20 a, i.e., the outer end of the solid polymerelectrolyte membrane 30 a. The second seal member 62 b contacts theouter end of the first metal separator 22. The third seal member 62 ccontacts the fourth metal separator 28 corresponding to the second metalseparator of the adjacent unit cell 12 b.

The first seal member 62 a forms an inner seal member for preventingleakage of the fuel gas, the second seal member 62 b forms anintermediate seal member for preventing leakage of the oxygen-containinggas, and the third seal member 62 c forms an outer seal member forpreventing leakage of the coolant.

The second membrane electrode assembly 20 b has the same structure asthe first membrane electrode assembly 20 a. At opposite ends of thesecond membrane electrode assembly 20 b in the direction indicated bythe arrow B, second uneven portions 38 b are provided in the same planeas the electrode surface. The second uneven portion 38 b at one end ofthe second membrane electrode assembly 20 b in the direction indicatedby the arrow B includes a fifth cutout 39 e, a fifth extension 37 e, asixth cutout 39 f, and a sixth extension 37 f from upper to lowerpositions. Further, the second uneven portion at the other end of thesecond membrane electrode assembly 20 b in the direction indicated bythe arrow B includes a seventh cutout 39 g, a seventh extension 37 g, aneighth cutout 39 h, and an eighth extension 37 h from upper to lowerpositions.

The third metal separator 26 has a second oxygen-containing gas flowfield 64 on a surface 26 a facing the second membrane electrode assembly20 b. At one end of the second oxygen-containing gas flow field 64 inthe direction indicated by the arrow B, a corrugated inlet section 63 ais formed, and likewise, at the other end of the secondoxygen-containing gas flow field 64 in the direction indicated by thearrow B, a corrugated outlet section 63 b is formed. The inlet section63 a and the outlet section 63 b protrude from the outer ends of thesecond membrane electrode assembly 20 b, respectively. Thus, when thesurface 26 b of the third metal separator 26 is stacked on the surface24 b of the second metal separator 24, the second coolant flow field 54is formed between the second metal separator 24 and the third metalseparator 26. The third metal separator 26 has an outer shape formingportion 65 having a predetermined uneven shape.

As shown in FIG. 7, the fourth metal separator 28 has a second fuel gasflow field 66 on a surface 28 a facing the second membrane electrodeassembly 20 b. The second fuel gas flow field 66 includes ridges 66 aand grooves 66 b, extending in the direction indicated by the arrow B.At opposite ends of the second fuel gas flow field 66, embossed sections66 c are formed.

As shown in FIG. 8, the first coolant flow field 44 is formed on asurface 28 b of the fourth metal separator 28, i.e., on the back of thesurface 28 a forming the second fuel gas flow field 66. When the fourthmetal separator 28 is stacked on the first metal separator 22, the firstcoolant flow field 44 is formed between the first metal separator 22 andthe fourth metal separator 28. At opposite ends of the first coolantflow field 44 in the direction indicated by the arrow B, a corrugatedinlet section 68 a and a corrugated outlet section 68 b are provided atouter positions.

The inlet section 68 a and the outlet section 68 b connect the coolantsupply passage 48 a and the coolant discharge passage 48 b to the firstcoolant flow field 44 through cutouts of the first metal separator 22.

The fourth metal separator 28 has two inlet holes 70 a and two outletholes 70 b at positions deviated from the two inlet holes 58 a and thetwo outlet holes 58 b of the second metal separator 24 in the stackingdirection. Further, the fourth metal separator 28 has three inlet holes72 a and three outlet holes 72 b at positions deviated from the threeinlet holes 60 a and the three outlet holes 60 b in the stackingdirection.

As shown in FIG. 7, a seal member 74 is formed integrally on a surface28 a of the fourth metal separator 28, around the second fuel gas flowfield 66. The seal member 74 includes a first seal (first seal member)74 a as an inner seal, a second seal (second seal member) 74 b as anintermediate seal positioned outside the first seal 74 a, and a thirdseal (third seal member) 74 c as an outer seal positioned outside thesecond seal 74 b. The first seal 74 a as the inner seal for preventingleakage of the fuel gas contacts the outer end of the solid polymerelectrolyte membrane 30 b of the second membrane electrode assembly 20b, the second seal 74 b as the intermediate seal for preventing leakageof the oxygen-containing gas contacts the outer end of the third metalseparator 26, and the third seal 74 c as the outer seal for preventingleakage of the coolant contacts the outer end of the second metalseparator 24 of the unit cell 12 a.

As shown in FIGS. 9 and 10, the second metal separator 24 and the fourthmetal separator 28 include a channel 75 connecting the oxygen-containinggas supply passage 46 a to the first oxygen-containing gas flow field 40and the second oxygen-containing gas flow field 64 in the presence ofthe third seals 62 c, 74 c. The channel 75 includes inlet holes (throughholes) 60 a, 72 a for the oxygen-containing gas. Likewise, as shown inFIGS. 11 and 12, the second metal separator 24 and the fourth metalseparator 28 include a channel 77 connecting the fuel gas supply passage50 a to the first fuel gas flow field 52 and the second fuel gas flowfield 66 in the presence of the third seals 62 c, 74 c. The channel 77includes inlet holes (through holes) 58 a, 70 a for the fuel gas.

A first channel 76 a is formed in the unit cell 12 a by the first unevenportion 38 a, and a second channel 76 b is formed in the unit cell 12 bby the second uneven portion 38 b.

As shown in FIG. 9, the first channel 76 a is formed at the first cutout39 a between the first metal separator 22 and the second metal separator24. The first channel 76 a includes a first oxygen-containing gas supplyconnection channel 78 a connecting the oxygen-containing gas supplypassage 46 a to the first oxygen-containing gas flow field 40. As shownin FIG. 11, the first channel 76 a includes a first fuel gas supplyconnection channel 79 a formed between the third extension 37 c and thesecond metal separator 24 for connecting the fuel gas supply passage 50a to the first fuel gas flow field 52.

As shown in FIG. 3, the first channel 76 a includes a firstoxygen-containing gas discharge connection channel 78 b formed at thefourth cutout 39 d between the first metal separator 22 and the secondmetal separator 24 for connecting the oxygen-containing gas dischargepassage 46 b to the first oxygen-containing gas flow field 40. The firstchannel 76 a includes a first fuel gas discharge connection channel 79 bformed between the second extension 37 b and the second metal separator24 for connecting the fuel gas supply passage 50 a to the first fuel gasflow field 52.

As shown in FIG. 10, the second channel 76 b includes a secondoxygen-containing gas supply connection channel 78 c formed at the fifthcutout 39 e between the third metal separator 26 and the fourth metalseparator 28 for connecting the oxygen-containing gas supply passage 46a to the second oxygen-containing gas flow field 64. As shown in FIG.12, the second channel 76 b includes a second fuel gas supply connectionchannel 79 c formed between the seventh extension 37 g and the fourthmetal separator 28 for connecting the fuel gas supply passage 50 a tothe second fuel gas flow field 66.

As shown in FIG. 3, the second channel 76 b includes a secondoxygen-containing gas discharge connection channel 78 d formed at theeighth cutout 39 h between the third metal separator 26 and the fourthmetal separator 28 for connecting the oxygen-containing gas dischargepassage 46 b to the second oxygen-containing gas flow field 64. Thesecond channel 76 b includes a second fuel gas discharge connectionchannel 79 d formed between the sixth extension 37 f and the fourthmetal separator 28 for connecting the fuel gas supply passage 50 a tothe second fuel gas flow field 66.

Operation of the fuel cell 10 having the above structure will bedescribed below.

As shown in FIG. 1, at the end plate 16 a, an oxygen-containing gas issupplied to the oxygen-containing gas supply passage 46 a, and a fuelgas such as a hydrogen-containing gas is supplied to the fuel gas supplypassage 50 a. Further, a coolant such as pure water or ethylene glycolis supplied to the coolant supply passage 48 a.

As shown in FIG. 6, the second metal separator 24 of the unit cell 12 ahas the three inlet holes 60 a connected to the oxygen-containing gassupply passage 46 a from the surface 24 b. Further, as shown in FIG. 8,the fourth metal separator 28 of the unit cell 12 b has the three inletholes 72 a connected to the oxygen-containing gas supply passage 46 afrom the surface 28 b.

In the structure, as shown in FIG. 9, some of the oxygen-containing gassupplied to the oxygen-containing gas supply passage 46 a flows throughthe inlet holes 60 a of the second metal separator 24 to the firstoxygen-containing gas supply connection channel 78 a on the surface 24a, and the oxygen-containing gas flows into the first oxygen-containinggas flow field 40 from the inlet section 41 a of the first metalseparator 22.

As shown in FIG. 10, in the unit cell 12 b, some of theoxygen-containing gas supplied to the oxygen-containing gas supplypassage 46 a flows through the inlet holes 72 a of the fourth metalseparator 28 to the second oxygen-containing gas supply connectionchannel 78 c on the surface 28 a, and the oxygen-containing gas flowsinto the second oxygen-containing gas flow field 64 from the inletsection 63 a of the third metal separator 26.

Further, as shown in FIG. 6, the second metal separator 24 has two inletholes 58 a connected to the fuel gas supply passage 50 a on the surface24 b. As shown in FIG. 8, the fourth metal separator 28 has two inletholes 70 a connected to the fuel gas supply passage 50 a on the surface28 b.

Thus, as shown in FIG. 11, some of the fuel gas supplied to the fuel gassupply passage 50 a flows through the inlet holes 58 a of the secondmetal separator 24 to the first fuel gas supply connection channel 79 aon the surface 24 a, and the fuel gas is supplied to the first fuel gasflow field 52 connected to the first fuel gas supply connection channel79 a.

Further, as shown in FIG. 12, some of the fuel gas supplied to the fuelgas supply passage 50 a flows through the inlet holes 70 a of the fourthmetal separator 28 to the second fuel gas supply connection channel 79 con the surface 28 a, and the fuel gas is supplied to the second fuel gasflow field 66 connected to the second fuel gas supply connection channel79 c.

Thus, as shown in FIG. 3, in the first membrane electrode assembly 20 a,the oxygen-containing gas supplied to the cathode 32 a and the fuel gassupplied to the anode 34 a are consumed in the electrochemical reactionsat electrode catalyst layers for generating electricity. Similarly, inthe second membrane electrode assembly 20 b, the oxygen-containing gassupplied to the cathode 32 b and the fuel gas supplied to the anode 34 bare consumed for generating electricity.

The oxygen-containing gas from the first oxygen-containing gas flowfield 40 of the unit cell 12 a flows from the outlet section 41 b to thefirst oxygen-containing gas discharge connection channel 78 b, and flowsto the surface 24 b through the outlet holes 60 b of the second metalseparator 24. Then, the oxygen-containing gas is discharged to theoxygen-containing gas discharge passage 46 b. Likewise, theoxygen-containing gas from the second oxygen-containing gas flow field64 of the unit cell 12 b flows from the outlet section 63 b to thesecond oxygen-containing gas discharge connection channel 78 d. Then,the oxygen-containing gas is discharged to the oxygen-containing gasdischarge passage 46 b through the outlet holes 72 b of the fourth metalseparator 28.

Further, the fuel gas from the first fuel gas flow field 52 of thesecond metal separator 24 flows into the first fuel gas dischargeconnection channel 79 b, and flows toward the surface 24 b through theoutlet holes 58 b. Then, the fuel gas is discharged to the fuel gasdischarge passage 50 b. Likewise, the fuel gas from the second fuel gasflow field 66 of the fourth metal separator 28 flows into the secondfuel gas discharge connection channel 79 d, and flows toward the surface28 b through the outlet holes 70 b. Then, the fuel gas is discharged tothe fuel gas discharge passage 50 b.

Further, as shown in FIG. 8, the inlet section 68 a and the outletsection 68 b connected to the first coolant flow field 44 are providedon the surface 28 b of the fourth metal separator 28. The inlet section68 a and the outlet section 68 b are provided at positions correspondingto the cutouts of the first metal separator 22.

In the structure, as shown in FIG. 13, the coolant supplied to thecoolant supply passage 48 a flows from the surface 28 b of the fourthmetal separator 28 to the first coolant flow field 44 formed between thefourth metal separator 28 and the first metal separator 22 through theinlet section 68 a. After the coolant flows through the first coolantflow field 44 for cooling the fuel cell 10, the coolant is dischargedfrom the surface 28 b to the coolant discharge passage 48 b through theoutlet section 68 b (see FIG. 3).

As shown in FIG. 6, the inlet section 56 a and the outlet section 56 bconnected to the second coolant flow field 54 are provided on thesurface 24 b of the second metal separator 24. The inlet section 56 aand the outlet section 56 b are provided at positions corresponding tothe cutouts of the third metal separator 26.

Thus, as shown in FIG. 14, the coolant supplied to the coolant supplypassage 48 a flows from the surface 24 b to the second coolant flowfield 54 formed between the second metal separator 24 and the thirdmetal separator 26 through the inlet section 56 a. After the coolantflows through the second coolant flow field 54, the coolant isdischarged through the outlet section 56 b from the surface 24 b intothe coolant discharge passage 48 b (see FIG. 3).

In the first embodiment, the outer dimensions of the first metalseparator 22 of the unit cell 12 a are smaller than the outer dimensionsof the second metal separator 24. In the first membrane electrodeassembly 20 a, the surface area of the anode 34 a is smaller than thesurface area of the cathode 32 a.

As shown in FIGS. 2 and 5, the first seal member 62 a which contacts theouter end of the solid polymer electrolyte membrane 30 a of the firstmembrane electrode assembly 20 a, the second seal member 62 b whichcontacts the outer end of the second metal separator 24, and the thirdseal member 62 c which contacts the fourth metal separator 28(substantially the same as the second metal separator 24) of theadjacent unit cell 12 b are formed integrally on the surface 24 a of thesecond metal separator 24.

Thus, in the second metal separator 24, simply by forming the seal onlyon the surface 24 a, in comparison with the case where the seal isformed also on the surface 24 b, the seal forming process issignificantly simplified, and carried out economically.

The first seal 62 a for preventing leakage of the fuel gas, the secondseal 62 b for preventing leakage of the oxygen-containing gas, and thethird seal 62 c for preventing leakage of the coolant are offset fromone another in the stacking direction indicated by the arrow A. Thus, incomparison with the structure in which at least any two of the fuel gasseal, the oxygen-containing gas seal, and the coolant seal areoverlapped with each other, reduction in the thickness of the fuel cell10 is achieved easily.

In the structure, each of the first seal 62 a, the second seal 62 b, andthe third seal 62 c has the sufficient height for suitably preventingleakage of the oxygen-containing gas, the fuel gas, and the coolant.Reduction in the thickness in the stacking direction is achieved. Theoverall size of the fuel cell 10 is reduced easily and reliably.

Further, the fuel gas flow field 52, the inlet holes 58 a, and theoutlet holes 58 b are sealed by triple seal structure including thefirst seal member 62 a, the second seal member 62 b, and the third sealmember 62 c. Thus, improvement in the fuel gas sealing performance isachieved suitably, and leakage of the fuel gas is prevented as much aspossible.

Further, in the first embodiment, the oxygen-containing gas supplypassage 46 a, the coolant supply passage 48 a, the fuel gas dischargepassage 50 b, the fuel gas supply passage 50 a, the coolant dischargepassage 48 b, and the oxygen-containing gas discharge passage 46 b arenot provided in the first metal separator 22. In the structure, adesired rigidity at the time of stacking the unit cells 12, and therigidity of the electrode surface required for assembling the unit cells12 a is maintained. Further, the weight reduction of the unit cell 12 ais achieved, and the overall weight of the fuel cell 10 is reducedeasily. Also in the unit cell 12 b, the same advantages as in the caseof the unit cell 12 a are achieved.

In the structure, the overall weight and size of the fuel cell 10 arereduced easily. In particular, at the time of stacking a large number ofunit cells 12 a, 12 b together to form the fuel cell 10 mounted in avehicle, the overall weight of the fuel cell 10 is reducedsignificantly, and improvement in the fuel economy is achieved reliably.

Further, in the unit cell 12 b, in effect, the first metal separator 22can be used as the third metal separator 26 by 180° rotation such thatthe first metal separator 22 and the third metal separator 26 areoriented oppositely. The second metal separator 24 can be used as thefourth metal separator 28 by 180° rotation such that the second metalseparator 24 and the fourth metal separator 28 are oriented oppositely.Thus, the fuel cell 10 has economical structure as a whole.

The first seal member 62 a, the second seal member 62 b, and the thirdseal member 62 c are provided on the second metal separator 24. Therounded front ends of the first seal member 62 a, the second seal member62 b, and the third seal member 62 c contact the flat outer end of thesolid polymer electrolyte membrane 30 a, the surface 22 a of the firstmetal separator 22, and the surface 28 b of the fourth metal separator28, respectively. In the structure, it is possible to reliably preventdecrease in the line pressure in the sealing regions, occurrence ofleakage, and deformation of the separators.

In the first embodiment, the seal member 62 is formed integrally withthe second metal separator 24. In the structure, unlike the structure inwhich the seal member 62 having a desired shape is formed beforehand,and then, the seal member 62 is joined to the second metal separator 24,operation of positioning the seal member 62 relative to the second metalseparator 24 is not required. Thus, the sealing structure is simplifiedsignificantly, and performance in the operation of assembling the fuelcell 10 is improved suitably.

As shown in FIGS. 2 and 5, the first seal 62 a which contacts the outerend of the solid polymer electrolyte membrane 30 a of the first membraneelectrode assembly 20 a, the second seal 62 b which contacts the outerend of the second metal separator 24, and the third seal 62 c whichcontacts the fourth metal separator 28 (substantially the same as thesecond metal separator 24) of the adjacent unit cell 12 b are formedintegrally on the surface 24 a of the second metal separator 24.

Thus, in the second metal separator 24, simply by forming the seal onlyon the surface 24 a, in comparison with the case where the seal isformed also on the surface 24 b, the seal forming process issignificantly simplified, and carried out economically.

Further, in the first metal separator 22, no seal forming operation isrequired. Thus, it is sufficient to form the seal member 62 only on thesecond metal separator 24. The seal forming operation is performedsimply and economically. The overall production cost of the fuel cell 10is reduced easily.

Further, in the first embodiment, the first uneven portions 38 a of thefirst membrane electrode assembly 20 a and the second uneven portions 38b of the second membrane electrode assembly 20 b which are adjacent toeach other in the stacking direction are offset from each other in thestacking direction. In the structure, the adjacent first channel 76 aand the second channel 76 b are not overlapped with each other in thestacking direction.

Specifically, for example, the first oxygen-containing gas supplyconnection channel 78 a connecting the first oxygen-containing gas flowfield 40 to the oxygen-containing gas supply passage 46 a is offset fromthe second oxygen-containing gas supply connection channel 78 cconnecting the second oxygen-containing gas flow field 64 to theoxygen-containing gas supply passage 46 a, in the stacking direction.Thus, it is possible to reduce the thickness of the unit cells 12 a, 12b in the stacking direction while maintaining the sufficient flow fieldheight and the sufficient seal height in the first oxygen-containing gassupply connection channel 78 a and the second oxygen-containing gassupply connection channel 78 c. Accordingly, reduction in the entiresize of the fuel cell 10 is achieved easily, and in particular, the sizeof the fuel cell stack mounted in a vehicle is suitably reduced.

FIG. 15 is a partial cross sectional view showing a fuel cell 80according to a second embodiment of the present invention. Theconstituent elements that are identical to those of the fuel cell 10according to the first embodiment are labeled with the same referencenumerals, and detailed description thereof will be omitted. Also inthird to sixteenth embodiments as described later, the constituentelements that are identical to those of the fuel cell 10 according tothe first embodiment are labeled with the same reference numerals, anddetailed description thereof will be omitted.

The fuel cell 80 is formed by stacking unit cells 82 a, 82 b alternatelyin the direction indicated by the arrow A. The unit cell 82 a is formedby sandwiching a first membrane electrode assembly 20 a between a firstmetal separator 83 and a second metal separator 84. The unit cell 82 bis formed by sandwiching a second membrane electrode assembly 20 bbetween a third metal separator 86 and a fourth metal separator 88.

The outer dimensions of the first metal separator 83 are larger than theouter dimensions of the second metal separator 84. The first metalseparator 83 has an inner seal (second seal member, third seal) 90 a forpreventing leakage of the coolant, an intermediate seal (first sealmember, first seal) 90 b provided outside the inner seal 90 a forpreventing leakage of the fuel gas, and an outer seal (third sealmember, second seal) 90 c provided outside the intermediate seal 90 bfor preventing leakage of the oxygen-containing gas.

The outer dimensions of the third metal separator 86 are larger than theouter dimensions of the fourth metal separator 88. The third metalseparator 86 has an inner seal (second seal member) 92 a for preventingleakage of the coolant, an intermediate seal (first seal member) 92 bprovided outside the inner seal 92 a for preventing leakage of the fuelgas, and an outer seal (third seal member) 92 c provided outside theintermediate seal 92 b for preventing leakage of the oxygen-containinggas.

In effect, the first metal separator 83 is used as the third metalseparator 86 by 180° rotation such that the first metal separator 83 andthe third metal separator 86 are oriented oppositely. In effect, thesecond metal separator 84 is used as the fourth metal separator 88 by180° rotation such that the second metal separator 84 and the fourthmetal separator 88 are oriented oppositely.

The inner seals 90 a, 92 a contact the outer ends of the fourth metalseparator 88 and the second metal separator 84, respectively. Theintermediate seals 90 b, 92 b contact the outer end of the solid polymerelectrolyte membrane 30 b of the second membrane electrode assembly 20 band the outer end of the solid polymer electrolyte membrane 30 a of thefirst membrane electrode assembly 20 a, respectively. The outer seals 90c, 92 c contact the outer ends of third metal separator 86 and the firstmetal separator 83 which are adjacent to each other.

In the second embodiment having the above structure, in the unit cell 82a, the inner seal 90 a, the intermediate seal 90 b, and the outer seal90 c are formed integrally on only one surface of the first metalseparator 83. Thus, seal forming process is significantly simplified,and carried out economically. Further, as the fuel gas seal structure,dual seal structure including the intermediate seal 90 b and the outerseal 90 c is adopted. Thus, the same advantages as in the case of thefirst embodiment are obtained. For example, the performance ofpreventing the leakage of the fuel gas is improved suitably.

FIG. 16 is a partial cross sectional view showing a fuel cell 80 aaccording to a third embodiment of the present invention. Theconstituent elements that are identical to those of the fuel cell 80according to the second embodiment are labeled with the same referencenumerals, and detailed description thereof will be omitted.

The fuel cell 80 a is formed by stacking unit cells 82 a, 82 balternately in the direction indicated by the arrow A. The unit cell 82a is formed by sandwiching a first membrane electrode assembly 20 abetween a first metal separator 83 a and a second metal separator 84 a.The unit cell 82 b is formed by sandwiching a second membrane electrodeassembly 20 b between a third metal separator 86 a and a fourth metalseparator 88 a.

The outer dimensions of the first metal separator 83 a are smaller thanthe outer dimensions of the second metal separator 84 a. A seal member90 is formed integrally with the second metal separator 84 a. The sealmember 90 includes an inner seal 90 a for preventing leakage of thecoolant, an intermediate seal 90 b provided outside the inner seal 90 afor preventing leakage of the fuel gas, and an outer seal 90 c providedoutside the intermediate seal 90 b for preventing leakage of theoxygen-containing gas.

The outer dimensions of the third metal separator 86 a are smaller thanthe outer dimensions of the fourth metal separator 88 a. A seal member92 is formed integrally with the fourth metal separator 88 a. The sealmember 92 includes an inner seal 92 a for preventing leakage of thecoolant, an intermediate seal 92 b provided outside the inner seal 92 afor preventing leakage of the fuel gas, and an outer seal 92 c providedoutside the intermediate seal 92 b for preventing leakage of theoxygen-containing gas.

In effect, the second metal separator 84 a is used as the fourth metalseparator 88 a by 180° rotation such that the second metal separator 84a and the fourth metal separator 88 a are oriented oppositely. Ineffect, the first metal separator 83 a is used as the third metalseparator 86 a by 180° rotation such that the first metal separator 83 aand the third metal separator 86 a are oriented oppositely.

The inner seals 90 a, 92 a contact the outer ends of the third metalseparator 86 a and the first metal separator 83 a, respectively. Theintermediate seals 90 b, 92 b contact the outer end of the solid polymerelectrolyte membrane 30 b of the second membrane electrode assembly 20 band the outer end of the solid polymer electrolyte membrane 30 a of thefirst membrane electrode assembly 20 a, respectively. The outer seals 90c, 92 c contact the outer ends of fourth metal separator 88 a and thesecond metal separator 84 a which are adjacent to each other.

In the third embodiment, operation of forming the seal member is notrequired for the first metal separator 83 a and the third metalseparator 86 a. Thus, the seal forming process is significantlysimplified, and carried out economically. The overall production cost ofthe fuel cell 80 a is reduced easily.

Further, in the third embodiment, the seal member 90 includes the innerseal 90 a for preventing leakage of the coolant, the intermediate seal90 b for preventing leakage of the fuel gas, and the outer seal 90 c forpreventing leakage of the oxygen-containing gas. The inner seal member90 a, the intermediate seal 90 b, and the outer seal 90 c are offsetfrom one another in the stacking direction. Likewise, the seal member 92includes the inner seal 92 a for preventing leakage of the coolant, theintermediate seal 92 b for sealing the fuel gas, and the outer seal 92 cfor preventing leakage of the oxygen-containing gas. The inner seal 92a, the intermediate seal 92 b, and the outer seal 92 c are offset fromone another in the stacking direction.

In the structure, it is possible to prevent leakage of the coolant, thefuel gas, and the oxygen-containing gas, and reduce the overall size ofthe fuel cell 80 a while maintaining the sufficient seal height in eachfluid channel.

FIG. 17 is a cross sectional view showing a fuel cell 100 according to afourth embodiment of the present invention.

The fuel cell 100 is formed by stacking a plurality of unit cells 102 inthe direction indicated by the arrow A. Each of the unit cells 102 isformed by sandwiching the membrane electrode assembly (electrolyteelectrode assembly) 104 between a first metal separator 106 and a secondmetal separator 108 (see FIGS. 17 and 18). The membrane electrodeassembly 104 includes a solid polymer electrolyte membrane 30, a cathode32, and an anode 34. The outer dimensions (surface areas) of the solidpolymer electrolyte membrane 30, the cathode 32, and the anode 34 arethe same.

The outer dimensions of the first metal separator 106 are smaller thanthe outer dimensions of the second metal separator 108. In effect, thefirst metal separator 106 has the same structure as the first metalseparator 22 of the first embodiment.

A seal member 110 is formed integrally with the second metal separator108. As shown in FIGS. 17 and 19, on the surface 24 a, the seal member110 includes a first seal (inner seal) 110 a formed around the firstfuel gas flow field 52, and a second seal (outer seal) 110 b.

The first seal 110 a contacts the outer end of the membrane electrodeassembly 104, and the second seal 110 b contacts the second metalseparator 108 of the adjacent unit cell 102 (see FIG. 17).

In the fourth embodiment having the above structure, instead of adoptingthe first membrane electrode assembly 20 a and the second membraneelectrode assembly 20 b including the electrodes having different sizes(referred to as a stepped MEA) according to the first embodiment, themembrane electrode assembly 104 including the electrodes having the samesize is adopted. Also in the structure, the same advantages as in thecase of the first embodiment are obtained.

FIG. 20 is a cross sectional view showing a fuel cell 112 according to afifth embodiment of the present invention.

The fuel cell 112 is formed by stacking a plurality of unit cells 114 ina direction indicated by the arrow A. The unit cell 114 includes amembrane electrode assembly 104, a first metal separator 106, and asecond metal separator 116. The outer dimensions of the first metalseparator 106 are smaller than the outer dimensions of the second metalseparator 116.

A first seal member 118 is formed integrally on the surface 24 a of thesecond metal separator 116, around the fuel gas flow field 52. The firstseal member 118 includes a first seal (inner seal) 118 a and a secondseal (outer seal) 118 b. The first seal 118 a contacts the outer end ofthe membrane electrode assembly 104. The second seal 118 b contacts asecond seal member 119 (described later) provided on the second metalseparator 126 of the adjacent unit cell 122.

The second seal member 119 is formed integrally with the surface 24 b ofthe second metal separator 126 around the coolant flow field 54. Thesecond seal member 119 includes a seal 119 a. The outer ends of themembrane electrode assembly 104 and the first metal separator 106 aresandwiched between the seal 119 a and the inner seal 118 a.

In the fuel cell 120 having the above structure, the same advantages asin the cases of the first to fourth embodiments are obtained.

FIG. 21 is a cross sectional view showing a fuel cell 120 according to asixth embodiment of the present invention.

The fuel cell 120 is formed by stacking a plurality of unit cells 122 ina direction indicated by the arrow A. The unit cell 122 includes amembrane electrode assembly 104, a first metal separator 124, and asecond metal separator 126. The outer dimensions of the first metalseparator 124 are smaller than the outer dimensions of the second metalseparator 126.

As shown in FIGS. 22 and 23, at substantially central positions of bothends of the first metal separator 124 in the longitudinal directionindicated by the arrow B, extensions 128 a, 128 b protrude toward thecoolant supply passage 48 a and the coolant discharge passage 48 b ofthe second metal separator 126, respectively.

The first metal separator 124 has a plurality of grooves 132 a, 132 b ona surface 22 a. The inlet side and the outlet side of theoxygen-containing gas flow field 40 are opened to the outside throughthe grooves 132 a, 132 b. As shown in FIG. 23, a curved seal member 134is provided around the oxygen-containing gas flow field 40 on thesurface 22 a of the first metal separator 124. Part of the seal member134 forms the grooves 132 a, 132 b.

As shown in FIG. 24, a coolant flow field 54 is formed in the surface 22b of the first metal separator 124. The extensions 128 a, 128 b havegrooves 136 a, 136 b connecting the coolant flow field 54 to the coolantsupply passage 48 a and the coolant discharge passage 48 b extendingthrough the second metal separator 126.

As shown in FIG. 25, a first seal member 138 and a second seal member140 are formed integrally on the surface 24 a of the second metalseparator 126 around the fuel gas flow field 52.

The membrane electrode assembly 104 is sandwiched between the first sealmember 138 and the seal member 134 of the first metal separator 124. Thesecond seal member 140 contacts the second metal separator 126 of theadjacent unit cell 122 (see FIG. 21).

As shown in FIG. 25, the first seal member 138 seals theoxygen-containing gas supply passage 46 a, the oxygen-containing gasdischarge passage 46 b, the coolant supply passage 48 a, and the coolantdischarge passage 48 b, while allowing the fuel gas supply passage 50 aand the fuel gas discharge passage 50 b to be connected to the fuel gasflow field 52.

Grooves 142 a are formed by the first seal member 138 between the fuelgas supply passage 50 a and the fuel gas flow field 52, and grooves 142b are formed by the first seal member 138 between the fuel gas dischargepassage 50 b and the fuel gas flow field 52.

As shown in FIG. 25, channels 143 a, 143 b connecting the fuel gassupply passage 50 a and the fuel gas discharge passage 50 b to the fuelgas flow field 52 are formed in the second metal separator 126. Thechannels 143 a, 143 b include grooves 142 a, 142 b, respectively.

Grooves 144 a and 144 b are formed between the first seal member 138 andthe second seal member 140. The grooves 144 a connect theoxygen-containing gas supply passage 46 a to the grooves 132 a of thefirst metal separator 124, and the grooves 144 b connect theoxygen-containing gas discharge passage 46 b to the grooves 132 b.

As shown in FIGS. 23 and 25, the second metal separator 126 has channels146 a, 146 b connecting the oxygen-containing gas supply passage 46 aand the oxygen-containing gas discharge passage 46 b respectively to theoxygen-containing gas flow field 40 of the first metal separator 124.The channels 146 a, 146 b include grooves 144 a, 144 b. The grooves 132a, 132 b are connected to the oxygen-containing gas flow field 40through the grooves 144 a, 144 b.

In the fuel cell 120 having the above structure, the first seal member138 and the second seal member 140 of one of the adjacent second metalseparators 126 tightly contact the other of the adjacent second metalseparators 126.

In the structure, the oxygen-containing gas supplied to theoxygen-containing gas supply passage 46 a of each unit cell 122 flowsinto the channel 146 a (see FIG. 25), and then, the oxygen-containinggas is supplied from the grooves 144 a of the second metal separator 126to the oxygen-containing gas flow field 40 through the grooves 132 a ofthe first metal separator 124 (see FIG. 23). The fuel gas consumed inthe reaction in the oxygen-containing gas flow field 40 is discharged tothe channel 146 b from the grooves 132 b, and flows through the grooves144 b to the oxygen-containing gas discharge passage 46 b.

As shown in FIG. 25, the fuel gas supplied to the fuel gas supplypassage 50 a of the unit cells 122 flows into the channel 143 a, and thefuel gas is supplied to the fuel gas flow field 52 through the grooves142 a. The fuel gas consumed in the fuel gas flow field 52 is dischargedfrom the grooves 142 b of the channel 143 b to the fuel gas dischargepassage 50 b.

Further, the coolant supplied to the coolant supply passage 48 a flowsthrough the grooves 136 a in the extension 128 a of the first metalseparator 24, and the coolant is supplied to the coolant flow field 54(see FIG. 24). Then, the coolant used for cooling each unit cell 122 isdischarged to the coolant discharge passage 48 b through the grooves 136b formed in the other extension 128 b.

In the sixth embodiment having the above structure, the same advantagesas in the cases of the first to fifth embodiments are obtained. Forexample, the outer dimensions of the first metal separator 124 arereduced significantly. It is possible to reduce the weight and the sizeof each of the unit cells 122.

FIG. 26 is a cross sectional view showing a fuel cell 150 according to aseventh embodiment of the present invention.

The fuel cell 150 is obtained by modifying the fuel cell 120 accordingto the sixth embodiment by adopting so called skip cooling structure inwhich the coolant flow field 54 is provided for every predeterminednumber of, e.g., every two membrane electrode assemblies 104.

The fuel cell 150 is formed by stacking a first metal separator 124, amembrane electrode assembly 104, a second metal separator 152, anothermembrane electrode assembly 104, a third metal separator (having thesame structure as the second metal separator 126) 154, another firstmetal separator 124, another membrane electrode assembly 104, anothersecond metal separator 152, another membrane electrode assembly 104, andanother third metal separator 154 successively in the directionindicated by the arrow A.

A fuel gas flow field 52 is formed on one surface of the second metalseparator 152 facing the membrane electrode assembly 104, and themembrane electrode assembly 104 is sandwiched between the second metalseparator 152 and the first metal separator 124. An oxygen-containinggas flow field 40 is formed on the other surface of the second metalseparator 152 facing the other membrane electrode assembly 104, and theother membrane electrode assembly 104 is sandwiched between the secondmetal separator 152 and the third metal separator 154.

In the seventh embodiment, the area of the coolant flow field 54 iseffectively reduced (by half). Thus, the overall dimension of the fuelcell 150 in the stacking direction is reduced significantly.

FIG. 27 is a cross sectional view showing a fuel cell 160 according toan eighth embodiment of the present invention.

The fuel cell 160 is formed by stacking unit cells 162 in the directionindicated by the arrow A. Each of the unit cells 162 is formed bysandwiching the membrane electrode assembly 104 between a first carbonseparator 164 and a second carbon separator 166.

The fuel cell 160 has substantially the same structure as the fuel cell120 according to the sixth embodiment. The outer dimensions of the firstcarbon separator 164 are smaller than the outer dimensions of the secondcarbon separator 166. A seal member 168 corresponding to the seal member134 is provided on the first carbon separator 164. A first seal 170 anda second seal 172 corresponding to the first seal member 138 and thesecond seal member 140 are provided on the second carbon separator 166.The first seal 170 and the second seal 172 are attached to the secondcarbon separator 166 after molding.

In the eighth embodiment having the above structure, the same advantagesas in the cases of the first to seventh embodiments are obtained. Forexample, the dimensions of the first carbon separator 164 are reduced asmuch as possible. It is possible to reduce the overall weight and sizeof the fuel cell 160.

The fuel cell 160 according to the eighth embodiment has substantiallythe same structure as the fuel cell 120 according to the sixthembodiment, and adopts the first carbon separator 164 and the secondcarbon separator 166. Also in the first to fifth embodiments and theseventh embodiments, carbon separators may be adopted.

FIG. 28 is an exploded perspective view showing a fuel cell 200according to a ninth embodiment of the present invention.

The fuel cell 200 is formed by stacking unit cells 202 a, 202 balternately in the direction indicated by the arrow A. The unit cell 202a is formed by sandwiching a first membrane electrode assembly 204 abetween the first metal separator 22 and the second metal separator 24,and the unit cell 202 b is formed by sandwiching a second membraneelectrode assembly 204 b between the third metal separator 26 and thefourth metal separator 28.

The first membrane electrode assembly 204 a has a substantiallyquadrangular shape as a whole. At opposite ends of the first membraneelectrode assembly 204 a in the direction indicated by the arrow B,first uneven portions 206 a forming the first channel 76 a are providedin the same plane as the electrode surface. The first uneven portion 206a at one end of the first membrane electrode assembly 204 a in thedirection indicated by the arrow B includes a first extension 37 a, afirst cutout 39 a, a second extension 37 b, and a second cutout 39 bfrom upper to lower positions. Further, the first uneven portion 206 aat the other end of the first membrane electrode assembly 20 a in thedirection indicated by the arrow B includes a third extension 37 c, athird cutout 39 c, a fourth extension 37 d, and a fourth cutout 39 dfrom upper to lower positions.

The second membrane electrode assembly 204 b has the same structure asthe first membrane electrode assembly 204 a. At opposite ends of thesecond membrane electrode assembly 204 b in the direction indicated bythe arrow B, a second uneven portions 206 b forming the second channel76 b are provided in the same plane as the electrode surface. The seconduneven portion 206 b at one end of the second membrane electrodeassembly 204 b in the direction indicated by the arrow B includes afifth cutout 39 e, a fifth extension 37 e, a sixth cutout 39 f, and asixth extension 37 f from upper to lower positions. Further, the seconduneven portion 206 b at the other end of the second membrane electrodeassembly 204 b in the direction indicated by the arrow B includes aseventh cutout 39 g, a seventh extension 37 g, an eighth cutout 39 h,and an eighth extension 37 h from upper to lower positions.

In the ninth embodiment having the above structure, the first unevenportions 206 a and the second uneven portions 206 b are offset from eachother in the stacking direction, and the same advantages as in the casesof the above embodiments are obtained.

FIG. 29 is an exploded perspective view showing a fuel cell 210according to a tenth embodiment of the present invention.

The fuel cell 210 is formed by stacking unit cells 212 a, 212 balternately in the direction indicated by the arrow A. The unit cell 212a is formed by sandwiching a first membrane electrode assembly 214 abetween the first metal separator 22 and the second metal separator 24,and the unit cell 212 b is formed by sandwiching a second membraneelectrode assembly 214 b between the third metal separator 26 and thefourth metal separator 28.

The first membrane electrode assembly 214 a has a substantiallyquadrangular shape as a whole. At opposite ends of the first membraneelectrode assembly 214 a in the direction indicated by the arrow B,first uneven portions 216 a forming the first channel 76 a are providedin the same plane as the electrode surface. The first uneven portion 216a at one end of the first membrane electrode assembly 104 a in thedirection indicated by the arrow B includes a first cutout 218 a, afirst extension 220 a, and a second cutout 218 b from upper to lowerpositions. Further, the first uneven portion 216 a at the other end ofthe first membrane electrode assembly 214 a in the direction indicatedby the arrow B includes a second extension 220 b and a third cutout 218c from upper to lower positions.

The second membrane electrode assembly 214 b has the same structure asthe first membrane electrode assembly 214 a. At opposite ends of thesecond membrane electrode assembly 214 b in the direction indicated bythe arrow B, second uneven portions 216 b forming the second channel 76b are provided in the same plane as the electrode surface. The seconduneven portion 216 b at one end of the second membrane electrodeassembly 214 b in the direction indicated by the arrow B includes afourth cutout 218 d and a third extension 220 c from upper to lowerpositions. The second uneven portion 216 b at the other end of thesecond membrane electrode assembly 214 in the direction indicated by thearrow B includes a fifth cutout 218 e, a fourth extension 220 d, and asixth cutout 218 f from upper to lower positions.

A first oxygen-containing gas supply connection channel 78 a is formedat the first cutout 218 a between the first metal separator 22 and thesecond metal separator 24. A first fuel gas supply connection channel 79a is formed between the first extension 220 a and the second metalseparator 24. A first oxygen-containing gas discharge connection channel78 b is formed at the third cutout 218 c between the first metalseparator 22 and the second metal separator 24. A first fuel gasdischarge connection channel 79 b is formed between the first extension220 a and the second metal separator 24.

A second oxygen-containing gas supply connection channel 78 c is formedat the fourth cutout 218 d between the third metal separator 26 and thefourth separator 28. A second fuel gas supply connection channel 79 c isformed between the fourth extension 220 d and the fourth metal separator28. A second oxygen-containing gas discharge connection channel 78 d isformed at the sixth cutout 218 f between the third metal separator 26and the fourth metal separator 28. A second fuel gas dischargeconnection channel 79 d is formed between the third extension 220 c andthe fourth metal separator 28.

In the tenth embodiment having the above structure, the first unevenportions 216 a and the second uneven portions 216 b are offset from eachother in the stacking direction, and the same advantages as in the casesof the above embodiments are obtained.

FIG. 30 is an exploded perspective view showing a fuel cell 230according to an eleventh embodiment of the present invention.

The fuel cell 230 is formed by stacking unit cells 232 a, 232 balternately in the direction indicated by the arrow A. The unit cell 232a is formed by sandwiching a first membrane electrode assembly 234 abetween a first metal separator 236 and a second metal separator 238,and the unit cell 232 b is formed by sandwiching a second membraneelectrode assembly 214 b between a third metal separator 240 and afourth metal separator 242.

The first membrane electrode assembly 234 a has a substantiallyquadrangular shape as a whole. At opposite ends of the first membraneelectrode assembly 234 a in the direction indicated by the arrow B,first uneven portions 236 a forming the first channel 76 a are providedin the same plane as the electrode surface. The first uneven portions236 a include a first extension 37 a, a first cutout 39 a, a secondextension 37 b, a second cutout 39 b, a third extension 37 c, a thirdcutout 39 c, a fourth extension 37 d and a fourth cutout 39 d providedat opposite ends of the first membrane electrode assembly 234 a in thedirection indicated by the arrow B.

The second membrane electrode assembly 234 b has the same structure asthe first membrane electrode assembly 234 a. At opposite ends of thesecond membrane electrode assembly 234 b in the direction indicated bythe arrow B, a second uneven portion 236 b forming the second channel 76b is provided along the electrode surface. The second uneven portion 236b includes a fifth cutout 39 e, a fifth extension 37 e, a sixth cutout39 f, and a sixth extension 37 f, a seventh cutout 39 g, a seventhextension 37 g, an eighth cutout 39 h, and an eighth extension 37 h. Thefirst extension 37 a to the eighth extension 37 h, and the first cutout39 a to the eighth cutout 39 h are inclined in a predetermined directionfrom the vertical direction.

The shapes of the first metal separator 236 to the fourth metalseparator 242 correspond to the shapes of the first membrane electrodeassembly 234 a and the second membrane electrode assembly 234 b.Likewise, the shapes of the oxygen-containing gas supply passage 46 a,the coolant supply passage 48 a, the fuel gas discharge passage 50 b,the fuel gas supply passage 50 a, the coolant discharge passage 48 b,and the oxygen-containing gas discharge passage 46 b correspond to theshapes of the first membrane electrode assembly 234 a and the secondmembrane electrode assembly 234 b.

In the eleventh embodiment having the above structure, the first unevenportion 236 a and the second uneven portion 236 b are offset from eachother in the stacking direction, and the same advantages as in the casesof the above described embodiments are obtained.

FIG. 31 is a cross sectional view showing a fuel cell 300 according to atwelfth embodiment of the present invention.

The fuel cell 300 is formed by stacking a plurality of unit cell 302 inthe direction indicated by the arrow A. The unit cell 302 is formed bysandwiching the membrane electrode assembly (electrolyte electrodeassembly) 304 between a first metal separator 306 and a second metalseparator 308 (see FIGS. 31 and 32). The membrane electrode assembly 304includes a solid polymer electrolyte membrane 30, a cathode 32, and ananode 34. The outer dimensions (surface areas) of the solid polymerelectrolyte membrane 30, the cathode 32, and the anode 34 are the same.

The outer dimensions of the first metal separator 306 are smaller thanthe outer dimensions of the second metal separator 308. In effect, thefirst metal separator 306 has the same structure as the first metalseparator 22 of the first embodiment.

A seal member 310 is formed integrally with the second metal separator308. As shown in FIGS. 31 and 33, on the surface 24 a, the seal member310 includes a first seal 310 a, a second seal 310 b, and a third seal310 c formed around the first fuel gas flow field 52.

The first seal 310 a for preventing leakage of the fuel gas contacts theouter end of the solid polymer electrolyte membrane 30, the second seal310 b for preventing leakage of the oxygen-containing gas contacts theouter end of the first metal separator 306, and the third seal 310 c forpreventing leakage of the coolant contacts the second metal separator308 of the adjacent unit cell 302 (see FIG. 31).

In the twelfth embodiment having the above structure, instead ofadopting the first membrane electrode assembly 20 a and the secondmembrane electrode assembly 20 b including the electrodes havingdifferent sizes (referred to as a stepped MEA) according to the firstembodiment, the membrane electrode assembly 304 including the electrodeshaving the same size as the solid polymer electrolyte membrane isadopted. Also in the structure, the same advantages as in the case ofthe above described embodiments are obtained.

FIG. 34 is a cross sectional view showing a fuel cell 320 according to athirteenth embodiment of the present invention.

The fuel cell 320 is formed by stacking a plurality of unit cells 322 ina direction indicated by the arrow A. The unit cell 322 includes amembrane electrode assembly 304, a first metal separator 324, and asecond metal separator 326. The outer dimensions of the first metalseparator 324 are substantially the same as the outer dimensions of thesecond metal separator 326.

As shown in FIGS. 35 to 37, an oxygen-containing gas supply passage 46a, a coolant supply passage 48 a, a fuel gas discharge passage 50 b, afuel gas supply passage 50 a, a coolant discharge passage 48 b, and anoxygen-containing gas discharge passage 46 b extend through the firstmetal separator 324 and the second metal separator 326 in the stackingdirection indicated by the arrow A.

As shown in FIGS. 34 and 36, an outer seal (third seal) 328 is formedintegrally on a surface 22 b of the first metal separator 324 around thecoolant flow field 44, along the outer end of the surface 22 b.

As shown in FIG. 37, a seal member 330 is formed integrally on a surface24 a of the second metal separator 326 around the fuel gas flow field52. The seal member 330 includes an inner seal (first seal) 330 a and anintermediate seal (second seal) 330 b. The inner seal 330 a contacts theouter end of the membrane electrode assembly 304 (see FIG. 34). Theintermediate seal 330 b contacts outer end of the first metal separator324 such that the membrane electrode assembly 304 is sandwiched betweenthe intermediate seal 330 b and the first metal separator 324.

As shown in FIG. 37, the seal member 330 seals the oxygen-containing gassupply passage 46 a, the oxygen-containing gas discharge passage 46 b,the coolant supply passage 48 a, and the coolant discharge passage 48 bwhile allowing the fuel gas supply passage 50 a and the fuel gasdischarge passage 50 b to be connected to the fuel gas flow field 52.

Grooves 342 a are formed by the seal member 330 between the fuel gassupply passage 50 a and the fuel gas flow field 52, and grooves 342 bare formed by the seal member 330 between the fuel gas discharge passage50 b and the fuel gas flow field 52. Grooves 344 a are formed near theoxygen-containing gas supply passage 46 a, and grooves 344 b are formednear the oxygen-containing gas discharge passage 46 b.

In the fuel cell 320 having the above structure, the oxygen-containinggas supplied to the oxygen-containing gas supply passage 46 a of eachunit cell 322 flows through the grooves 344 a of the second metalseparator 326 (see FIG. 37), and the oxygen-containing gas is suppliedto the oxygen-containing gas flow field 40 of the first metal separator324 (see FIG. 35). The oxygen-containing gas consumed in the reaction inthe oxygen-containing gas flow field 40 flows through the grooves 344 bof the second metal separator 326, and the oxygen-containing gas isdischarged to the oxygen-containing gas discharge passage 46 b.

As shown in FIG. 37, the fuel gas supplied to the fuel gas supplypassage 50 a of each unit cell 322 flows through the grooves 342 a, andthe fuel gas is supplied to the fuel gas flow field 52. The fuel gasconsumed in the fuel gas flow field 52 is discharged to the fuel gasdischarge passage 50 b through the grooves 342 b.

The coolant supplied to the coolant supply passage 48 a is supplied tothe coolant flow field 54 (see FIG. 36). After the coolant is used forcooling each unit cell 322, the coolant is discharged to the coolantdischarge passage 48 b.

In the thirteenth embodiment, the outer seal 328 for preventing leakageof the coolant is provided in the outer end of the first metal separator324, and the inner seal 330 a for preventing leakage of the fuel gas,and the intermediate seal 330 b for preventing leakage of theoxygen-containing gas are provided in the second metal separator 326.The outer seal 328, the inner seal 330 a, and the intermediate seal 330b are offset from one another in the stacking direction. Thus, the sameadvantages as in the cases of the above described embodiments areobtained. For example, the dimension of the fuel cell 320 in thestacking direction is reduced as much as possible, and the overall sizeof the fuel cell 320 is reduced easily.

FIG. 38 is a cross sectional view showing a fuel cell 350 according to afourteenth embodiment of the present invention.

The fuel cell 350 includes a plurality of unit cells 352. Each of theunit cells 352 is formed by stacking a membrane electrode assembly 304between a first metal separator 354 and a second metal separator 356.

As shown FIG. 39, the seal member 358 is formed integrally on thesurface 22 a of the first metal separator 354 around theoxygen-containing gas flow field 40. The seal member 358 includes aninner seal (second seal) 358 a and an intermediate seal (first seal) 358b. The inner seal 358 a contacts the outer end of the cathode 32 of themembrane electrode assembly 304. The intermediate seal 358 b is formedaround the membrane electrode assembly 304, and contacts the secondmetal separator 356 (see FIG. 38).

An outer seal (third seal) 360 is formed integrally on the surface 24 bof the second metal separator 356 around the coolant flow field 54,around the outer end of the surface 24 b.

In the fourteenth embodiment, the inner seal 358 a for preventingleakage of the oxygen-containing gas, the intermediate seal 358 b forpreventing leakage of the fuel gas, and the outer seal 360 forpreventing leakage of the coolant are offset from one another in thestacking direction. Thus, the same advantages as in the cases of theabove described embodiments are obtained.

FIG. 40 is a cross sectional view showing a fuel cell 370 according to afifteenth embodiment of the present invention.

The fuel cell 370 includes unit cells 372 each including a membraneelectrode assembly 374, a first metal separator 324, and a second metalseparator 326. As shown in FIGS. 40 and 41, the membrane electrodeassembly 374 includes an anode 34 c having a surface area smaller thanthe surface areas of the solid polymer electrolyte membrane 30 and thecathode 32.

In the fifteenth embodiment having the above structure, the sameadvantages as in the case of the fuel cell 320 according to thethirteenth embodiment are obtained.

FIG. 42 is a cross sectional view showing a fuel cell 380 according to asixteenth embodiment of the present invention.

The fuel cell 380 includes unit cells 382 each formed by sandwiching amembrane electrode assembly 304 between the first metal separator 306and the second metal separator 384 (see FIGS. 42 and 43). A seal member330 having an inner seal 330 a and an intermediate seal 330 b is formedintegrally on the surface 24 a of the second metal separator 384. Anouter seal (third seal) 386 is formed integrally on the surface 24 b ofthe second metal separator 384. The outer seal (third seal) 386 contactsthe surface 24 a of the adjacent second metal separator 384 to preventleakage of the coolant (see FIG. 42).

In the sixteenth embodiment having the above structure, the inner seal330 a, the intermediate seal 330 b, and the outer seal 386 are offsetfrom one another in the stacking direction, and the same advantages asin the cases of the above described embodiments are obtained.

FIG. 44 is a cross sectional view showing a fuel cell 390 according to aseventeenth embodiment of the present invention.

The fuel cell 390 includes unit cells 392 each having a membraneelectrode assembly 304, a first carbon separator 394, and a secondcarbon separator 396. A first seal 398 as an inner seal member and asecond seal 400 as an intermediate seal member are formed on the surface24 a of the second carbon separator 396. The first seal 398 contacts theouter end of the membrane electrode assembly 304 for preventing leakageof the fuel gas, and the second seal 400 contacts the first carbonseparator 394 such that the membrane electrode assembly 304 issandwiched between the second seal 400 and the first carbon separator394 for preventing leakage of the oxygen-containing gas.

A third seal 402 for preventing leakage of the coolant is formed on thesurface 24 b of the second carbon separator 396. The third seal 402contacts the outer end of the first carbon separator 394 of the adjacentunit cell 392 for preventing leakage of the coolant.

The first seal 398, the second seal 400, and the third seal 402 areoffset from one another in the stacking direction, and the sameadvantages as in the cases of the above described embodiments areobtained. For example, the overall dimension of the fuel cell 390 in thestacking direction is reduced as much as possible.

The invention claimed is:
 1. A fuel cell comprising stacked unit cellseach including an electrolyte electrode assembly and a first separatorand a second separator sandwiching the electrolyte electrode assembly,the electrolyte electrode assembly including an anode, a cathode, and anelectrolyte interposed between the anode and the cathode, a fuel gasflow field for supplying a fuel gas along the anode and anoxygen-containing gas flow field for supplying an oxygen-containing gasalong the cathode being formed in the unit cells, a coolant flow fieldfor supplying a coolant along an electrode surface being formed betweenthe unit cells at intervals of every predetermined number of the unitcells, wherein at least the first separator or the second separatorincludes a first seal surrounding the fuel gas flow field for preventingleakage of the fuel gas, a second seal surrounding the oxygen-containinggas flow field for preventing leakage of the oxygen-containing gas, anda third seal surrounding the coolant flow field for preventing leakageof the coolant; and all of the first seal, the second seal and the thirdseal are offset from one another when viewed from a stacking direction.2. A fuel cell according to claim 1, wherein the first seal forms aninner seal, the second seal forms an intermediate seal, and the thirdseal forms an outer seal.
 3. A fuel cell according to claim 1, whereinthe third seal forms an inner seal, the first seal forms an intermediateseal, and the second seal member forms an outer seal.
 4. A fuel cellaccording to claim 1, wherein the second seal forms an inner seal, thefirst seal forms an intermediate seal, and the third seal member formsan outer seal.
 5. A fuel cell according to claim 1, wherein a fuel gassupply passage, a fuel gas discharge passage, an oxygen-containing gassupply passage, an oxygen-containing gas discharge passage, a coolantsupply passage, and a coolant discharge passage extend through an outerend of at least one of the first separator and the second separator inthe stacking direction.